A polymer electrolyte fuel cell comprises a solid polymer membrane (Nafion, manufactured by E. I. du Pont de Nemours and Company; Dow membrane, manufactured by Dow Chemical Company; etc.) that functions as an electrolytic membrane, porous graphite papers disposed on both sides of the electrolytic membrane, and a platinum alloy catalyst serving as an electrode catalyst supported on the surface of the papers. The cell is so structured that a porous graphite plate having grooves as passageways for gas and a flat separator are arranged on each outer side of the graphite papers in this order, or a flat separator having grooves as passageways for gas on each outer side of the graphite papers.
Gas-impermeability to oxygen and hydrogen, electric conductivity, thermal conductivity, mechanical strength, acid resistance, and the like are required of the flat separators. In addition to the requirements of the flat separator, the separator having grooves is also required to have high dimensional accuracy for the gas passageways. Furthermore, the separators should be thin, because, for example, around 100 to 600 separators are layered to construct one fuel cell. Specifically, it is demanded that the number of bipolar separators used be reduced by making their thickest portions 2 mm or less and by forming grooves on both sides thereof, and it is also demanded that the thin portions thereof be made as thin as possible (for example, about 0.8 mm) by forming the grooves as deeply as possible.
A separator of this kind is manufactured in a manner such that a mixture of phenol resin; a binder made of a petroleum-based pitch or coal-based pitch or the like that exhibits a high carbonization yield; and carbon powder are molded into a flat plate. The resulting flat plate is subjected to carbonization or graphitization in a non-oxidizing atmosphere, obtaining a carbonaceous or graphite flat plate. Then, grooves are formed on the obtained plate by machining. For example, Japanese Unexamined Patent Publication No. 1992-214072 discloses a method to obtain a carbon material for a fuel cell, which comprises a step of molding a carbonaceous composition including a carbonizable or graphitizable binder, carbon fiber, and carbonaceous powder grains, wherein the carbonaceous powder grains composed of powder grains being average grain sizes of 25 to 75 μm, powder grains being 75 to 125 μm, and powder grains being 125 to 175 μm; and a step of graphitizing the resulting molded carbonaceous composition. This publication also discloses that the carbonaceous composition contains 10 to 75 parts by weight of carbon fiber and 50 to 150 parts by weight of carbonaceous powder grains, based on 100 parts by weight of binder. In the Example, a molded sheet having a thickness of 2 mm was graphitized.
However, when the carbonaceous composition is carbonized or graphitized, its gas-impermeability is degraded, and warping and cracks tend to appear on the carbon sheet material, reducing the yield. Furthermore, it is difficult to make the sheet thin while also improving the machining processability of the graphite carbon material. Moreover, forming grooves by machining a graphite carbon material makes the carbon material very expensive.
WO99/49530 proposes a method for manufacturing a separator for a fuel cell by subjecting a resin composition containing a non-carbonaceous resin and a conductive agent to injection molding or compression molding. Japanese Unexamined Patent Publication No. 1987-260709 discloses a carbon molded article that comprises 10 to 30 wt. % of a thermosetting resin and graphitized meso-carbon microbeads having a particle size of 50 μm or less. This publication also discloses that a thin sheet having a thickness of 0.8 mm was obtained.
Japanese Unexamined Patent Publication No. 1985-246568 discloses a method for manufacturing a ribbed separator for a fuel cell. The method comprises a step of press molding a mixture of 25 to 30 wt. % of a phenol resin and 70 to 75 wt. % of graphite powder under a temperature at which resin does not graphitize. This publication also discloses that a conductive plate having a thickness of 2 mm was obtained by using graphite powder classified to a particle size in the range of 100 to 325 mesh (about 150 to 44 μm). In the molding process for a ribbed molded article disclosed in the publication, it is necessary to have a resin amount of 25 wt. % to improve moldability, thereby obtaining a molded article having a volume electric resistance on the order of 10−2 Ω·cm.
Japanese Unexamined Patent Publication No. 1984-213610 discloses a carbon molded article constructed by 10 to 25 wt. % of a thermosetting resin and graphite powder, in which the aspect ratio of the graphite powder is 3 or less, the maximum grain size of graphite powder is 104 μm, 10 to 80% of the powder has a grain size of 50 μm or less, and the molded article has an electric specific resistance of 0.03 Ω·cm or less. In this publication, the aspect ratio is set to 3 or less by applying trituration, since oblate particles decrease moldability.
However, all of the compositions described above have insufficient moldability, and therefore, when the resin amount is reduced (for example, a composition having a resin amount of less than 25 wt. %), it becomes difficult to obtain a uniform molded article that is very thin (for example, a thickness of 2 mm or less) and provided with grooves, ribs, or manifolds. It is particularly difficult to obtain a thin and uniform molded article having yet thinner portions in bumps, dips, grooves, etc.
An object of the present invention is to provide a conductive composition, by which a thin molded article having a homogeneous constitution can be obtained, even when the molded article has a complicated structure such as bumps and dips or grooves that function as ribs, manifolds, etc., and to provide a polymer electrolyte fuel cell separator and a polymer electrolyte fuel cell using the separator.
Another object of the present invention is to provide a conductive composition that makes it possible to obtain a separator for a polymer electrolyte fuel cell that is excellent in gas-impermeability, electric conductivity, thermal conductivity, mechanical strength, acid resistance, and the like at a low cost without subjecting to a carbonization or graphitization process.
Still another object of the present invention is to provide a conductive composition that makes it possible to obtain a separator for a polymer electrolyte fuel cell and that has a high electric conductivity, thermal conductivity, and like excellent properties, and grooves (passageways for gas) with high dimensional accuracy by subjecting the composition to only a molding process and no a machining process, and to provide a separator for a polymer electrolyte fuel cell using the above-described composition and a polymer electrolyte fuel cell using the separator.
Yet another object of the present invention is to provide a conductive composition exhibiting high molding flowability and moldability even when the content of resin is reduced that makes it possible to obtain a molded article having a high conductivity, and to provide a separator for a polymer electrolyte fuel cell using the composition and a polymer electrolyte fuel cell using the separator.